Hydraulic synchronizing circuits are commonly known. Usually, one master cylinder unit and one slave cylinder unit are used. Each cylinder includes a piston connected to a piston rod extending through the primary chamber of the cylinder, so that the master cylinder unit and the slave cylinder unit work in a pulling manner. The diameter of the slave cylinder unit is slightly bigger than the diameter of the master cylinder unit, as it corresponds to the surface of the piston rod of the slave cylinder unit. A pump is provided which sucks in hydraulic medium and pumps the hydraulic medium into the primary chamber of the master cylinder unit via a control valve. The secondary chamber of the slave cylinder unit is connected to atmosphere to prevent compression inside the secondary chamber of the slave cylinder unit in case of an activation of the synchronizing unit. The master cylinder unit and the slave cylinder unit are arranged single-acting. Single-acting means that a return movement of the cylinders only results from external forces. A control valve has a position in which it enables the hydraulic medium to flow back into the tank from the primary chamber of the master cylinder unit. The two cylinders are connected in series by the second conduit connecting the secondary chamber of the master cylinder unit to the primary chamber of the slave cylinder unit.
Another hydraulic synchronizing circuit is known in which two identical cylinders are used. The cylinders each include two identical piston rods of identical diameters on both sides of their piston. The master cylinder unit is connected in series to the slave cylinder unit. The secondary chamber of the master cylinder unit is connected to the primary chamber of the slave cylinder unit by the second conduit. The secondary chamber of the slave cylinder unit is not connected to the atmosphere. A return conduit is provided which leads back to the tank via the control valve.
It is disadvantages in both synchronizing circuits that, in any case, a separate tank has to be provided. The dimensions of the tank have to chosen so that the tank is capable of at least holding the volume of the fluid to be pumped into the primary chamber of the master cylinder unit to attain the maximum stroke. When the cylinders have identical piston rods on both sides of their piston, the tank can generally be designed smaller since hydraulic medium is pumped back into the tank from the secondary chamber of the slave cylinder unit each time the cylinder is activated. Nevertheless, the separate arrangement after tank is an additional constructive requirement which is costly. Additionally, cylinders having piston rods on both sides of there piston are more complicated and more expensive.